First line of defense:
Adequate exterior lighting, some on timers, some on motion detectors.
Second line of defense:
Early warning system consisting of Emma the Tactical Terrier and Toffy the Wonder Beagle.
Third line of defense:
Alarm system and integrated home automation. (the lights turn on for the zone which is triggered)
Forth line of defense:
3 separate hidden doors/rooms. These are not fully-armored "safe rooms', rather just someplace where you'd be difficult to find when hiding.
Fifth line of defense:
There is quite a bit of weaponry. Some of it is almost always carried on my person or that of my wife. Everyone in the household is proficient in varying degrees appropriate to their age.